Of course 'favourite' is dependant on my mood, what I'm feeling like I need at that time. For instance, Warriors' Gate and Frontios are my 'chicken soup' Doctor Who stories, ones I tend to enjoy watching when I want some comfort. I love both The Invasion and Daleks' Master Plan for all sorts of reasons, though I think Kevin Stoney's performance in each is awesome fun to watch. Happiness Patrol and Web Planet are both the sorts of stories that only Doctor Who would attempt, and I love the insanity of both.
This is the problem, whatever I pick as my favourite will be completely different this time tomorrow, and different again next week. But, if I have to pick one...
Revelation of the Daleks is a comedy, one that features Daleks, funerals, cannibalism, Davros, direction by Graeme Harper, and acting from William Gaunt, Clive Swift, Eleanor Bron, and Alexie Sayle. Needless to say, it's a bizarre black comedy, with moments of weird horror. Its flaws are many but they don't really matter because when it works, it's wonderful.
There's the disgraced Knight of Oberon and his foul-smelling servant, Davros stating that telling people his new food supplement was made of their dead might lead to 'consumer resistance,' Mr. Jobel's vanity, and of course, Daleks being killed by highly-directional ultrasonic beams of rock 'n roll.
I think my favourite moment of all the silliness is a small one right near the end. Davros has had his hand blown off. He's about to be taken away by the Daleks to stand trial. As he's about to leave, the Doctor moves as if he's about to shake Davros' hand, and Davros raises his bandaged stump for a moment and then brings it back down in annoyance as if he's just remembered he no longer has a hand.